1. Field of the Invention
The present invention is broadly concerned with improved RF antenna assemblies used as a part of an induction or other type of heating apparatus in order to establish and maintain RF communication between the heating apparatus and an object being heated having a peripheral-mounted RF transponder. More particularly, it is concerned with such antenna assemblies, as well as overall heating systems and combinations thereof including heatable objects, making use of the improved RF antenna assemblies. The preferred RF antenna assemblies comprise multiple antenna loops cooperatively defining a substantially continuous RF communication zone outboard of a cooking hob.
2. Description of the Prior Art
Several prior art induction heating systems have been developed which use RF communications between a transmitter/receiver forming a part of the induction heater, and a radio frequency transponder (e.g., a RFID tag) associated with the object to be heated by the induction heater. Such RF communications include transponder feedback that is use by the induction heater to alter and/or control the heating of the object. The transmitter/receivers of such systems also include an antenna designed to interrogate the transponder and to receive information therefrom. The position of the antenna relative to the work coil of the induction heater in these systems is important in establishing and maintaining the necessary RF communication, and in allowing the user some freedom of placement of the object while it is being heated.
For example, U.S. Pat. No. 6,320,169, incorporated by reference herein in its entirety, describes an induction heating system having a RFID antenna located at the center of the cooking hob, i.e., in the center of the heater's work coil. In this type of system the object being heated can have a RFID tag affixed to the object's symmetry position, typically in the geometric center of the object. This symmetry position for both the RFID antenna and the RFID tag allows use of standard RFID antennas typically constructed of planar spiral or other geometric shape traces printed on a rigid substrate, with associated on-board capacitors) and other electronic components. This symmetry orientation allows the object to be heated to be rotated through a full 360 E angular orientation while atop the hob, without loss of RFID communication.
However, many heatable objects are designed to be heated to a temperature by a cooking/warming hob that exceeds the maximum operating temperature range of the RFID tag (usually 85° C., and sometimes 125° C. for microchip-based RFID tags, or possibly even higher for chipless RFID tags, resonant tag labels, planar LC resonators, printed RFID tags, or other chipless sensors such as the SENS-10, each sold by TagSense, Inc. of Cambridge, Mass.). Hence, it is often impractical to place the RFID tag or other transponder in a heatable portion of the object such as the center symmetry position. This is especially true in connection with cooking vessels or utensils, which are commonly subjected to very high heating temperatures.
One response to this problem is to mount the transponder or RFID tag on the periphery of an object subjected to high heating/warming temperatures, thereby reducing the heat load on the transponder or tag. The first known attempt to use a periphery-mounted RFID tag on a cooking vessel is described in U.S. Pat. No. 6,953,919. This patent discloses the use of a RFID tag preferentially located in the vessel's handle, remote from the heatable portion of the vessel, and thus allowing the tag to operate and survive at the ambient or slightly elevated temperatures of the vessel handle. However, this patent teaches that the RFID reader antenna can only maintain RF communication with the handle-mounted RFID tag through a limited angular rotation of the vessel. Indeed, this patent discloses that the RFID reader antenna preferably covers only a quadrant of the periphery of the work coil. Consequently, where the RFID tag is handle-mounted, the vessel must be maintained in a relatively small range of angular positions, else the necessary RF communication between the tag and reader will be lost. This presents a significant problem to the user, i.e., casual or even professional users may accidentally move the vessel handle out of the range of the RFID antenna during food preparation. Moreover, many users wish to place vessel handles in various different orientations for ease of food preparation or to ensure that a given handle is not inadvertently contacted, resulting in spillage.
Thus, designers of warming/cooking devices such as induction cooktops have recognized that the ability to allow a user to have the freedom to rotate vessel handles through a wide angular range during heating/cooking is an important feature. Attempts have been made to address this problem in several published patent applications. For instance, Japanese Publication No. 2006-344453, entitled “Heating Cooking Device” recognizes the handle placement/antenna problem, and provide the user with an aural or visual alarm which is activated if RF communications are lost between an induction cooking range antenna and the associated vessel handle-mounted RFID tag.
Japanese Publication No. 2006-294372, entitled “Heating Cooker” describes cooking systems wherein the communication area of the RFID system is varied by changing the electrified areas of the antenna. In other words, more or less of the traces of the antenna circuit are powered, based upon the stage of the cooking operation. Thus, before cooking is initiated, and before the pan handle is placed within the antenna zone, the smallest antenna area is electrified, thus making the antenna read range narrower so as to force the user to place the pan handle in the proper location relative to the electrified antenna area. Then, after cooking begins, more outlying antenna traces are electrified so as to have a wider reading area, and thus reduce the number of reading errors as the user rotates the pan handle during the cooking sequence. However, this system is inherently very complex, still only allows for RF communications over a limited portion of the periphery of the hob, and does not provide a full answer to the problem.
No known prior art describes any structure or means which provides a RF antenna forming a part of a heating device for use with cookware, servingware, or other heatable objects equipped with peripheral-mounted RF transponders, wherein the object being heated can be rotated through substantially 360° and/or radially displaced without loss of RF communication between the transponder and heating device. Accordingly, there is a real and unsatisfied need in the art for an improved antenna useful with a variety of heating devices and which establishes a substantially continuous RF communication zone outboard of and substantially surrounding the hob(s) of the heating device, thereby allowing a user to rotate an object being heated having a peripheral RF transponder to virtually any desired angular position without communication loss.